As a conventional draft device, a roller draft device is known which is composed of a plurality of roller pairs that pull a sliver while nipping and feeding the sliver downstream.
Further, the roller draft device has a plurality of roller pairs each composed of a top roller and a bottom roller, and disposed in a direction in which a sliver is fed. For the drafting, the roller pairs are rotated at different rotation speeds so that the rotation speed of one roller pair is slightly higher than that of the preceding one. A draft cradle is disposed which temporarily holds and supports each top roller so that the top roller can be contacted with and separated from the corresponding bottom roller.
The top roller is a rubber roller or a roller covered with rubber. The bottom roller is made of metal. Further, a force that grips the bottom and top rollers is important in reliably drafting the sliver. Accordingly, the top roller is pressed and abutted against the fixedly installed metallic bottom roller.
For example, various spinning devices are well known to manufacture a spun yarn by drafting a sliver (a bundle of fibers). Recently, spinning machines are known which comprise a hollow guide shaft (a hollow spindle) and a spinning nozzle and which can carry out spinning at a high speed of at least 300 m/min. In spinning machines feeding a sliver or a spun yarn at the high speed and represented by the recent ones, drafting is also carried out at a high speed to increase the rotation speed of each draft roller. In particular, the peripheral speed of a front roller that is a feeding roller located at a draft terminal increases significantly. This also increases the adverse effect of associated air currents generated around a peripheral surface of the front roller, which rotates at the high speed. Thus, disadvantageously, the quality of a spun yarn obtained varies and is unstable.
Thus, for the high-speed spinning device, a front top roller for a draft device has been disclosed which has its opposite ends cut by a large length to reduce an effective roller width to about half in order to prevent fibers from being diffused by associated air currents. Further, a draft device has been disclosed in which thin grooves are formed in the front top roller to provide channels for the associated air currents in order to prevent the fibers from being spread by the associated air currents.
The rotation speed of the front roller has been sharply increasing consistently with the speed at which the sliver or spun yarn is fed. Thus, the high speed rotation has often affected even draft devices using a well-known front roller of a grooved roller type.
Specifically, for relatively low spinning speeds used in the prior art, a draft roller composed of a grooved bottom roller and top roller made of rubber has been considered to be suitable for preventing the effect of associated air currents. However, it has been found that at increased feeding speeds, the associated air currents are diffused in a horizontal direction (the axial direction of the roller), thus affecting the physical properties of the spun yarn.
Thus, in the draft device intended to prevent the diffusion of fibers caused by associated air currents, a space or a groove portion is formed to allow the associated air currents to escape in a direction in which the paired front rollers are rotated; the associated air currents otherwise escape in the axial direction from the vicinity of the nip point between the paired front rollers, which rotate at high speed. However, this technique does not positively utilize the associated air currents to align the fibers with one another in the feeding direction, the fibers otherwise escaping in the horizontal direction.
It is an object of the present invention to provide a draft device comprising draft rollers which, when a draft device composed of a plurality of draft roller pairs is used, can reduce the adverse effect of associated air currents resulting from the rollers rotating at high speed while maintaining a gripping force and can effectively utilize the associated air currents to suppress the spread of a bundle of fibers.